Device for measuring in three dimensions a topographical shape of an object

ABSTRACT

A device for measuring in three dimensions a topographical shape of an object. The device comprises an arrayed confocal imaging system having a confocal topographical mask provided for converting light produced by a light source into an array of small spots. The mask being mounted on a scanning member provided for moving the mask over successive positions over a predetermined distance. The device further comprises a confocal objective provided for mapping at successive object-position-in-focus the array of small spots output at said successive positions. The confocal objective may be mounted at a fixed position within said device.

BACKGROUND OF THE INVENTION

The present invention relates to a device for measuring in threedimensions a topographical shape of an object, said device comprising anarrayed confocal imaging system having a light source provided forilluminating said object and a light path diverging optical element,provided for diverging a path of illuminating light output by said lightsource and a path of reflected light reflected by said object, saidconfocal imaging system further comprises a confocal topographical maskprovided for converting illuminating light coming out from said lightpath diverging optical element into an array of small spots, saidconfocal imaging system further comprises a confocal objective providedfor orienting said illuminating light towards said object and saidreflected light towards said confocal topographical mask in order toform a confocal image, said device further comprises a photoelectricsensor member provided to receive said reflected light having passedsaid confocal topographical mask and deflected by said light pathdiverging optical element and to convert the latter light into anintensity value, said device comprising also a scanning member on whichsaid confocal topographical mask is mounted, said scanning member beingprovided for moving said confocal topographical mask over successivepositions over a predetermined distance in order to modify a relativedistance in a predetermined direction between said object and theobject-position-in-focus, said device further comprises an imageprocessor connected to said photoelectric sensor member and provided forforming a confocal response signal and calculating said object shapefrom said confocal images acquired at different relative distances insaid predetermined direction between said object and theobject-position-in-focus by said photoelectric sensor member, saidconfocal objective being provided for mapping at successiveobject-position-in-focus said array of small spots output at saidsuccessive positions.

The invention also relates to a method for measuring in three dimensionsa topographical shape of an object.

Such a device and method are known from EP-A-0679864. In the knowndevice and method the light source outputs a path of illuminating lightwhich crosses the light path diverging optical element formed by ahologram and a lens array and reaches the confocal topographical maskcomprising a pinhole array. The path of illuminating light which isformed by an array of small spots after passing the mask, ends at theobject to be measured. At the location of the object the incident lightspots are reflected and the reflected light crosses the mask to reachthe light path diverging element. The latter deflects the reflectedlight towards the photoelectric sensor member, where the incident lightis sensed and further processed by the image processor in order todetermine a three dimensional shape of the considered object. In orderto determine the third dimension, means are provided for modifying therelative distance in the z-direction between the object and theobject-position-in-focus. In the known device, the latter means areformed by a set-up comprising a scanning member on which the confocaltopographical mask, the light path diverging optical element, the sensormembers and the confocal objective are mounted. The movement of thewhole set-up in the z-direction causes the object-position-in-focus toshift in the z-direction over a predetermined distance. Accordingly therelative distance in the z-direction between the object and theobject-position-in-focus changes, thus enabling to determine the thirddimension.

A drawback of the known device and method is that the confocaltopographical mask as well as the light path diverging optical element,the sensor members and the confocal objective are all moved together inthe z-direction in order to modify the relative distance between theobject and the object-position-in-focus. The movement of such arelatively heavy set-up requires some power and is not the mostappropriate choice for high speed on-line determination.

SUMMARY OF THE INVENTION

It is an object of the present invention to realise a device and/or amethod for measuring, in three dimensions, a shape of an object, whichdevice uses an arrayed confocal imaging system operative at high speedand enabling a fast measurement.

For this purpose, a device according to the present invention ischaracterised in that said confocal objective is a 3D confocal objectivemounted at a fixed position within said device. By using a fixed 3Dconfocal objective only the topographical mask is moved over thesuccessive positions. Since the topographical mask is much lighter thanthe whole set-up, the movement requires much less power than the oneaccording to the prior art. Consequently a faster movement is achievedwhich is more suitable for high speed on-line determination. It shouldbe noted that the prior art does not suggest the skilled person to keepthe confocal objective fixed. On the contrary, the alternativeembodiment illustrated in FIG. 11 of the prior art teaches to keep themask fixed and to move a part of the confocal lens. Starting from thisprior art, the skilled person is thus led away from fixing the confocalobjective.

Preferably said 3D confocal objective is being designed in such a manneras to limit spherical aberration and coma in order to keep imagingerrors in said confocal images at a maximum of two pixels. In order toobtain a good image quality at the level of the image processor, someconstraints have to be imposed on the confocal objective. Theseconstraints are met by setting a limit to the spherical aberration andcoma of the confocal lens.

A first preferred embodiment of a device according to the invention ischaracterised in that said confocal topographical mask is formed by amicrolens array mounted on said scanning member and said device furthercomprises a single pinhole located at a focal point of saidphotoelectric sensor member.

Preferably said illumination source comprises a high-density LED arraywith a further microlens bonded on top of it. In such a mannersufficient light intensity is provided.

A second preferred embodiment of a device according to the presentinvention is characterised in that a linear polarizer is applied intosaid light path at an illumination side of said diverging opticalelement on which other side a quarter-wave plate and an analyser areapplied. In such a manner “noise” light originating from reflectionswhich do not contribute to the reflected light to be measured, isconsiderably reduced.

A third preferred embodiment of a device according to the presentinvention is characterised in that said scanning member comprises ascanning signal generator provided for generating a series of scanningsignals indicative of said successive positions, said scanning signalgenerator being connected to said image processor for supplying saidscanning signals thereto, said photoelectric sensor member comprising afirst array of sensing elements, said image processor comprises a secondarray of processing elements, each processing element of said secondarray being each time connected to a number of sensing elements of saidfirst array, said image processor being provided for receiving each timewithin a same series of scanning signals successive ones of saidintensity values, each of said processing elements being provided fordifferentiating said successive intensity values among each other andretaining those intensity values forming said confocal response signal,said image processor being provided for associating to those intensityvalues forming said confocal response signal, those scanning signalsrepresenting those positions having led to said confocal responsesignal. By supplying the scanning signals to the processing elements,the latter operate synchronously with the scanning member, thus enablinga fast processing. The presence of a first array of sensing elements anda second array of processing elements enables to split the sensing ofthe incident light and the processing of the sensed light, thusincreasing the total process speed and making the device even moresuitable for a fast measurement. By differentiating the intensityvalues, the confocal response signal can be retained as it has thehighest intensity value. The amount of data to be processed is thereforreduced. The shape of the object can quickly be determined, since thescanning signal, corresponding to the highest value, is easilyrecognised due to the synchronous operation of the scanning member andthe image processor.

A fourth preferred embodiment of a device according to the invention ischaracterised in that a sampling member is mounted between said firstarray of sensing elements and said second array of processing elements,said sampling member being provided for sampling at a predeterminedsampling rate, intensity values generated by said sensing elements andoutput at parallel read-out gates thereof, each of said processingelements having a memory element provided for storing each of theintensity values sampled within a same series of scanning signals, eachof said processing elements being provided for determining a maximumintensity value by interpolating the intensity values stored each timein a same one of said memory elements. This enables to reduceconsiderably the amount of intensity values to be treated withoutsubstantially affecting the reliability.

A fifth preferred embodiment of a device according to the presentinvention is characterised in that said first and second array have atleast a same number of elements, each processing element being providedfor storing as a stored intensity value in an associated memory elementan initial intensity value, said processing elements being provided forcomparing, under control of each of the subsequent scanning signals, ifthe current intensity value is higher than the stored intensity valueand for overwriting the stored intensity value if the latter is lowerthan the current intensity value, said processing elements being alsoprovided for storing upon each storage operation the current one of thescanning signals. The highest intensity value is easily and quickly aswell as reliably determined.

Preferably said scanning member comprises a voice-coil actuatorconnected to said confocal topographical mask and is provided forimposing a vertical movement to said mask. A voice-coil actuator enablesa precise and reliable movement of the mask.

The invention will now be described in more detail with reference to thedrawings, showing different embodiments of a device according to thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 illustrates the optical principle as used in the device accordingto the invention;

FIG. 2 illustrates the light intensity of the reflected light incidenton the sensor member as a function of the movement of the mask for onesensing element;

FIG. 3 illustrates schematically the device where the mask is formed bya microlens array and a single pinhole;

FIG. 4 illustrates schematically the device where use is made of apolariser, a quarter-wave plate and an analyser;

FIG. 5 illustrates a possible confocal z-response signal;

FIGS. 6 and 7 illustrate embodiments of the image sensing elements andprocessing elements;

FIG. 8 illustrates by means of a flowchart a method for distinguishing alocal and a global maximum from each other;

FIG. 9 illustrates schematically an objective as a component of thedevice where use is made of a two-part objective; and

FIG. 10 shows a table with an example of a memory content.

In the drawings a same reference number has been assigned to a same oranalogous element.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a device and also a method formeasuring, in three dimensions, a topographical shape of an object, i.e.3D surface profiles, with a speed compatible to the one used inindustrial manufacturing processes. For on-line applications within amanufacturing process, the time allotted for such a measurement is about0.1 sec and preferably less. The objects to be measured generally havedimensions xyz=50×50×3 mm, wherein z indicates the height direction. Therequired accuracy is in the few micron range, in particular whenelectronic components are considered. It will however be clear that thepresent invention is not limited to the examples given here before.Nevertheless and for the sake of clarity, the referred example will beused throughout the description.

The device according to the invention comprises an arrayed confocalimaging system. The basic principle of a confocal imaging system isdescribed in “confocal scanning optical microscopy and related imagingsystems” of T. R. Corle and G. S. Kino published by Academic Press 1996.A light point source, defined by a pinhole, is used to illuminate theobject through an objective. The light reflected from a point on theobject is imaged by the objective back to the pinhole. If the pinholeand the spot on the sample point are at their confocal points, thereflected light is efficiently passing the pinhole to a detector locatedat the other side. If the object moves out of focus, the confocalrelation is not valid and the reflected light is defocused at thepinhole and hence does not pass through it to a detector located on theother side.

The confocal imaging system is independent of details of the surfaceprofile, structure, texture, roughness, reflectivity or colour variationand enables to be implemented without mechanical xy scanning. Instead ofchanging the optical path distance between the object surface and theobjective, the device according to the invention keeps the optical pathdistance fixed but let a confocal topographical mask move up and down inthe z-direction. Furthermore, the device uses a 2D image sequencesensing and processing by means of a photoelectric sensor member,preferably using CMOS technique and an image processor.

A basic problem overcome by the present invention is to deal with anenormous data rate. As a numerical example, assume 1000×1000 parallelconfocal array in xy. In order to achieve few micron accuracy around1000 or at least 100 images sectioning the 3 mm z interval stated aboveare needed. In order to achieve the maximum time of 0.1 sec, one needsto acquire and process said 100 to 1000 images within these 0.1 sec,i.e. there should be dealt with a rate of 1,000 to 10,000 images/sec ormore, each image comprising 1 M pixels. This is a data rate of 1-10billion pixels per second minimum, preferably more. The presentinvention presents a system capable of acquiring and processing theimages with this data rate.

FIG. 1 illustrates the optical principle as used in the device andmethod according to the invention. A light source 1 emits a light beam10 provided for illuminating an object 6. The light beam 10 crossesfirst a projection objective 2, provided to convert the light beam 10into a parallel illuminating light beam 11. A light path divergingoptical element 3, for example formed by a beamsplitter, is placed intothe light path of the illuminating light beam 11 and is provided fordiverging the light path of beam 11 and a path 14 of light reflected bythe object. As can be seen in FIG. 1, the illuminating light beam 11 isnot affected by element 3.

The illuminating light beam 11 illuminates in a homogeneous manner apinhole array 4, which is part of a confocal topographical mask,provided for converting the illuminating light beam 11 leaving element 3into an array of small spots. The pinhole array 4 comprises a pluralityof pinholes, for example a quadratic matrix of 1000×1000 pinholes, eachhaving a diameter of 5μ on a 50×50 mm plane i.e. with a pitch of 50 μ.The small spots emerging from each of these pinholes (for the sake ofclarity only a single spot has been designed) is focused by means of alarge aperture objective 5, in such a manner, that a focused beam 13 isincident on the object 6. Since those small spots are incident on theobjective 5, the latter maps them on the object. This objective 5preferably has telecentric properties to obtain identical illuminationproperties across the objects xy-dimensions (horizontal orientationwithin the drawing). If the z positions (vertical position withindrawing) of pinhole array 4, objective 5 and a surface point on theobject 6 are such that the arrangement is confocal, i.e. a spot ismapped onto a sharp point at the specific object surface location, alarge amount of the reflected light from the object surface 6 travelsback the identical path, i.e. 13 and 12, into the original pinhole. Thelight beam 14 emerging from the other side of the pinhole 4 is deflectedby beamsplitter 3 into a camera objective 7. The light is then focused15 onto the photoelectric sensor member. The sensor 8 is controlled andread out by image processor electronics 9.

If due to the surface profile of object 6 there is no surface at theposition of focal point, only very little light will be reflected backinto the pinhole by the out-of-focus surface area 6. Therefore, thepixel within sensor 8 will receive maximum light and form the confocalresponse signal at that relative location of pinhole array 4, objective5, and surface 6, where the confocal relation is met (confocal opticalprinciple). This is illustrated by the solid line shown in FIG. 2.

In order to obtain the third dimension it will be necessary to modify ina predetermined direction (the z-direction in the illustrated example)the relative distance between the object 6 and theobject-position-in-focus.

Those means for modifying the relative distance comprise a scanningmember 16 on which the confocal topographical mask 4 is mounted. Thescanning member 16 enables an up and down movement, shown by arrow 17,of the mask or pinhole array. The scanning member therefor moves themask over successive positions in the predetermined direction and over apredetermined distance of for example 3 mm.

It has to be noted that in the device or method according to the presentinvention only the confocal topographical mask 4 is moved during thescanning movement. The other components such as the objective remainfixed.

The scanning member further comprises a scanning signal generator 18provided for generating a series of scanning signals indicative of thesuccessive positions reached by the mask during its movement. Thedifferent scanning signals within a series of scanning signals thusrepresent the different positions. The scanning signal generator isfurther connected to the sensor member and the image processor in orderto supply the scanning signals to them.

As can be seen in FIG. 1, the objective 5 is placed downstream,considered in the illuminating path direction of the moving mask. Sincethe mask moves, the array of small spots output by the mask will alsomove in the predetermined direction. The objective 5 will thus map thedifferent small spots at different height onto differentobject-in-focus-positions of the object.

An advantage of changing the optical distance between the confocal lens5 and the mask 4 by means of the scanning member, is that the mask has alower mass than for example the Nipkow disc described in U.S. Pat. No.5,737,084. This lower mass enables a higher acceleration resulting in afaster linear movement instead of a rotation. Moreover, the deviceaccording to the invention is easier to build because the workingdistance between the confocal lens 5 and the object 6 remains fixed,since the confocal lens 5 remains fixed within the device. The scanningmember is fully enclosed in the device and the confocal lens can easilybe changed for changing the field of view.

In the present embodiment, the scanning member is for example formed bya voice-coil actuator. Due to the small mass of the pinhole array, afast movement is possible. Therefore, several mm may be traversed withinonly a few milliseconds. This implements the fast z-scanning requiredfor in-line measurement in order to achieve fast measurement times.Since a smaller aperture objective 7 is used, all of the movement ofpinhole array 4 remains within the depth of focus, so that thecollection of light into sensor pixel 8 is not affected by thismovement.

The pinhole array undergoes preferably a continuous up or down movement,and each scanning signal within that series corresponds to a particularz-position of the pinhole array 4.

An illuminating light beam 10, output by the light source, will thuscross the projection objective 2 and the beamsplitter 3 in order toreach the mask 4. A spot belonging to light beam 12 and leaving one ofthe pinholes will be focused by the objective 5. If the actual positionof the mask 3 during its movement and the point P on object 6, on whichthe focused spot is incident, are in focus, the focused spot will betotally reflected in point P. If on the other hand they are not infocus, only a small amount or even no light will be reflected. Thereflected light will then be diverged by the beamsplitter 3 and reachthe objective lens 7 and the sensor member 8.

Considering a fixed pixel within sensor member 8, the light intensityseen by this pixel will thus generally be very low. As an exception,when the confocal condition between the z-positions of pinhole array 4,objective 5, and surface 6 is met, the confocal response signal will beformed because there will be a maximum of light seen in this pixel. Byrecording the position of the pinhole array 4 at the point in time wherea maximum light intensity is observed by the sensor pixel 8, ameasurement of the corresponding z co-ordinate of surface point P isachieved. The intensity as a function of z-position 16 seen by a givenpixel could look like the solid line shown in FIG. 2.

Instead of using a pinhole array for the confocal topographical mask 4,it is also possible to place a microlens in each pinhole, facing theside of the light source. This would improve the light efficiency of thedevice. The microlenses would focus all incident light into the pinholesand collimate all light returning through the pinholes into the sensordirection. Insufficient collimated light, light incident between themicrolenses or noise light reflected by the object or optical elementsdownstream of the pinhole array could be blocked by the latter array.

According to an alternative embodiment the confocal topographical maskis formed by a microlens array at the position of the pinhole array 4.In this embodiment a single pinhole 20 is located at the focal point ofthe camera objective 7-1 and 7-2, as shown in FIG. 3. Any ray of light25 not passing through the microlens' focal point would hit that singlepinhole plate 20 outside the pinhole and not reach the sensor. For thepurpose of this discussion, the camera objective, FIG. 3, is imagined asa two-stage objective, the first half 7-1 imaging onto the focal pointand the second half 7-2 from the focal point to the sensor member 8(telecentric objective). The limited aperture of this objective replacesthe function of the array of pinholes at the microlens plate. Ifmicrolenses are used, the pinhole array could be omitted since the smallacceptance range of the microlenses arising from the small numericalaperture of the illumination/camera objectives would serve as effectivepinholes.

As explained in the latter section, the light source defining lightfiltering and detecting element defined by microlenses, eventuallycombined with additional pinholes, can have many (static and non-static)configurations of which a few have been described. It is used as ascanning member to determine the confocal topography of the object'ssurface.

The confocal objective 5 has to be suitable to operate over the wholeoptical scan such as realised by the scanning member over a 3D volumewith a depth of about 3 mm in order not to affect the image quality.This requires an appropriate designed lens 5, which is capable to keeplens distortions low within a 3D volume. Lenses are usually designed foroptimal distortions within a 2D-plane orthogonal to the optical axis andat a fixed optical distance from the lens. Scanning an object's surfaceis therefore mostly done by moving the object in the z-direction or bychanging the optical distance between object and objective lens.

The confocal objective 5 used in the device or method according to thepresent invention is therefor formed by a 3D confocal objective. Sharpimaging of a flat 2D object requires the fulfilment of the Abbe sinecondition. In ideal paraxial geometrical optics an infinite objectvolume can be adequately mapped into an infinite image volume. Howeverthis property can not be maintained for larger numerical apertures,except for a magnification equal to one. These conditions are describedin the article “The Abbe sine condition and related imaging conditionsin geometrical optics” of Joseph Braat, Fifth International TopicalMeeting on Education and Training in Optics, Delft, 1997. The latterarticle being incorporated by reference within the present description.

In order to achieve a suitable large 3D volume imaging at largernumerical apertures, it would be necessary to comply with the Abbe sinecondition as well as with the Herschel condition. This is howevercontradictory except for the case of magnification equal to one. Forpractical purpose however some spherical aberration is acceptable.

Detailed numerical simulation shows that up to magnification of +/−5 andnumerical apertures of +/−0.3, the spherical aberration and coma can bekept sufficiently small in order to keep imaging errors up to a maximumof two pixels at the level of the sensor.

Referring back to the Braat article, the constraints to be imposed onthe 3D confocal lens 5 can be derived by optimising the axial andlateral extent of the imaging volume as expressed in equation (19) ofthe article, in such a manner that coma and spherical aberration remainwithin the set limit. The optimisation can be achieved by computersimulation. The parameters which are available for tuning are themaximum numerical aperture and the wavelength of the used light source.A larger wavelength is favourable in order to maximise the imagingvolume of a 3D confocal objective.

An alternative embodiment consists of using a set of two 2D-optimizedstandard lenses and fixed positions for the microlens plate 4 and theobject 6. Such a two stage confocal lens is shown in FIG. 9 andcomprises a moving part 70, having a focal length f1, and a fixed part71, having a focal length f2, with a telecentric aperture 72 in between.Such a set-up is able to scan the object's surface by moving the lens 70closest to the object i.e. by changing the optical distance between theobject and the lens 70. The light reflected back from the object'ssurface is converted into a collimated beam by this lens 70. Thiscollimated reflected beam can be imaged by the second lens 71, fixed inposition, onto the fixed microlens plate 4. Advantages of such aconfocal lens set-up are that a very simplified optical design (standard2D optimized lenses can be used) is obtained when compared with a single3D confocal lens, a fixed object position and fixed microlens position.This is however at the expense of a higher mass which has to be movedand which movement is outside the device and is therefor exposed to theenvironment. Furthermore, it is more difficult to exchange the lenses iffor example different field-of-views would be required.

In the embodiment shown in FIG. 9 the confocal topographical mask isformed by a microlens array comparable to the one illustrated in FIG. 3.Consequently with the embodiment of FIG. 9, an analogous set-up as theone illustrated in FIG. 3, is chosen for the camera objective (notshown), i.e. a single pinhole located between a two-stage objective.

The device is very demanding regarding light energy. The amount of lightto be supplied increases with the speed at which the object isdisplaced, i.e. shorter exposure times, and with the volume to beinspected. Furthermore, the amount of “signal” light i.e. the lightreflected from the surface of the object and reaching the sensor member8 by the optical path designated by 13, 12, 14 and 15, strongly dependson the reflective properties of the object. The required maximum amountof light power for the collimated light beam 11, taking into accountlight losses at various components of the optical path, can be of theorder of a few tens of Watt under the above mentioned requirements ofinspection speed and volume. Due to the requirement for very lowstand-still times, the integration time at which the sensor member hasto operate is short, so a high light intensity is required to overcomethis problem.

A common light source used in the prior art is a Halogen bulb. However,an extremely intense light source (several hundreds of Watt) would berequired to deliver the required light energy. Only a small part of theemitted light can be used to produce a proper collimated light beam 11.In order to provide well-defined focal points at the microlenses 4, thenumerical apertures of the illumination optics as well as the cameraoptics should be rather small, around 0.01. Such highly inefficientlight source would suffer from low operating lifetime.

Another alternative design for a light source 1 would be a high-densityLED array. This is manufactured by means of bonding bare LED dies onto asubstrate, which in turn is bonded onto a heat sink. In this manner avery large number, e.g. 10,000 LED dies can be put into a 50 mm by 50 mmarea, providing the very large light intensity required. To increase thelight efficiency of this source a second microlens plate can be bondedatop the LED array, using a transparent adhesive or molding substance.This would serve to collimate a larger fraction of the light emitted bythe LEDs in the measurement path.

A third alternative light source, providing even higher lightintensities and efficiencies is an array of laser diodes with the lightcoupled into a fibre-optic bundle. The other end of the bundle wouldserve as a high-intensity light point source, which can effectively beconverted into a properly defined collimated beam. Laser speckle ishowever a disadvantage introduced by most common laser sources. Thecoherence of the light output source can be reduced by using an array ofindividual gain-guided diode laser sources, by allowing controlled butrelatively large temperature variations of the lasing media (resultingin a broader wavelength range) and coupling the light into a fibrebundle, which allows efficient mixing of the different light ray pathlengths.

Other possibilities to further reduce the coherence of the light emittedby the source would be a current modulation applied to the laser diodesto induce mode-hopping or by attaching a piezo- or voicecoil vibrator tothe fibre bundle and shake it. This continuous movement of the fibrebundle would also change the path lengths of the light rays. Thefrequency range is selected such that sufficient averaging is obtainedduring the time that one z-slice is recorded. During the image exposuretime, i.e. during the recording of one complete movement of the scanningmember, the voice-coil actuator 16 moves continuously in thez-direction. The image is formed by light integration in the camera 8, 9and this, itself, already constitutes a phase averaging.

A major source of “noise” light consists of unwanted reflections fromthe various elements in the optical path. A significant portion of thenoise light originates from the reflection at the microlens and/orpinhole array 4. This source of noise light can be eliminated asillustrated in FIG. 4 by using a linear polariser 21 in the illuminationpath following the objective 2 a quarter-wave (λ/4) plate 22 on theobject side of pinhole array 4 and an analyser 23 in the camera lightpaths. The polariser 21 is positioned at the illumination side of thebeamsplitter 3. It produces linearly polarised light, e.g. in they-direction.

The beamsplitter can be optimised by means of a proper optical coatingto be highly transmissive for light polarised in the y-direction andstrongly reflective for light polarised in the x-direction. Reflectionpreserves the polarisation direction. Reflected light from opticalelements downstream of the polariser 21 and before the quarter-waveplate 22 is linearly polarised in the y-direction. Because thebeamsplitter is made highly transmissive for this polarisation, it willnot significantly reflect towards the camera 8, 9.

The linear analyser 23 in front of the camera, having optimaltransmission properties for light polarised in the x-direction, willfurther reduce the light with this polarisation. Light reflected fromthe surface of the object is circularly polarised by the quarter-waveplate 22. Part of this light travels back along the path 13 and willpass the quarter-wave plate a second time but in opposite direction. Thesignal light will undergo a polarisation change from circular to linearin the x-direction. The beamsplitter 3 will efficiently reflect thislight towards the camera 8, 9. The reflected light has the properorientation in polarisation to pass the analyser with minimalattenuation. This set-up (polariser, beamsplitter, quarter-wave plateand analyser) would serve to reduce unwanted reflections directly fromthe illumination side of the microlens array or pinhole array 4 intocamera 8, 9 and enhance the signal-to-noise ratio. Polariser andanalyser could be combined within the beamsplitter 3.

Further reduction of unwanted reflection can be achieved by:

arranging the various optical elements with a small deviation fromperpendicular to optical axis;

by applying anti-reflective coatings to the optical elements.

Another source of noise is cross-talk between light from neighbouringpinholes or microlenses. Its effect can e.g. be reduced by the use of asmall aperture objective 7 in front of the camera.

Returning to FIG. 1, the processing of the light reflected by the object6 is realised by the sensor member 8 and the associated image processor9. Although the operation of the sensor member 8 and the image processor9 will be described by referring to reflected light obtained asdescribed here before, it will be clear that the sensor member and theimage processor could also be used for processing reflected lightobtained by another confocal imaging system. The sensor member and theimage processor have to deal with the reflected light leaving each ofthe pinholes or microlenses of the mask 4. In the numerical example of a1000×1000 pinhole array matrix the sensor member 8 would need an arrayof 1000×1000 sensors, preferably formed by CMOS sensors, in order tosimultaneously handle the reflected light. Moreover, since the mask 4 ismoving, a sequence of 100 to 1000 z-slice images are generated during asingle sweep operation of the scanning member. Since the whole scanningpath of the mask 4 is about 3 mm long, this operation can easily beachieved within 100 msec, leading to a data rate of 1 to 10 billionpixels per second to be processed by the image processor. Even fasteracquisition is possible in conjunction with still higher data rates,probably another factor of 10 can be achieved with a fast actuator andan extremely intense light source, requiring 10-100 billion pixels persecond data rates. In this example a 1:1 objective 5 is assumed.

In order to process the different images acquired during a scanningoperation of the mask, the photoelectric sensor member comprises a firstarray of sensing elements 8 ₁₁-8 _(nm) and the image processor comprisesa second array of processing elements 9 as shown in FIGS. 6 and 7. Eachprocessing element of the second array being each time connected to anumber of sensing elements of the first array. In the exampleillustrated in FIG. 6, there is a 1-1 relation between the sensingelements 8 _(ij) (1≦i≦n; 1≦j≦m) and the processing elements 9 _(ij),i.e. to each sensing element there is connected a processing element,whereas in the example illustrated in FIG. 7 each processing element 9-I(1≦l≦p) is connected to a group of sensing elements.

Consider now each pixel produced by a reflected light beam incident onone of the sensing elements independently and assume one processingelement for each pixel. In this case during one scan of the scanningmember 16, this pixel will see low light intensity most of the time andone maximum of intensity somewhere within the time interval used for thez-scan, approximately 100 msec as an example. The processing requiresthe determination of the point in time during the scan where the maximumoccurred and thus where its confocal response signal was formed. Theresult per pixel is not any greyvalue or light intensity, but thetimestamp indicating when the maximum occurred. The timestamp isindicative for the scanning member position. For each pixel, there issufficient time for processing: assuming 1000 z-slices, the data rate isof only one value per 100 μsec, or 10 kHz since each pixel is consideredindependently. In essence, 1 million parallel processing elements areused in order to deal with the 10 billion pixel per second data rate.Due to the high z-sampling rate no special algorithm (e.g.interpolation) is required to determine the z-position at maximumintensity with an accuracy of a few microns.

There are many ways to implement this parallel processing. What isrequired is that the scanning signal generator 18 supplies the generatedscanning signals to the processing elements 9. The scanning signalgenerator comprises for example a clock or time register and the clockpulses output by this clock are converted into scanning signals enablinga time controlled movement of the scanning member 16. By supplying thescanning signals to both the processing elements and the scanningmember, the processing elements can follow the movement of the mask andestablish a link between an obtained maximum light intensity and theposition of the mask. The scanning signal is for example supplied in theform of a digital signal, for example a 10 bit signal, enabling toidentify at least 1000 z-positions of the mask.

A possible processing algorithm executed by each image processingelement comprises for example the following steps:

1. Initialise running maximum to zero or another initial intensity valueand store as a stored intensity value the initial intensity value into amemory element of the processing element;

2. Under control of each of the subsequent scanning signals within asame series of scanning signals, compare the stored intensity value withthe intensity value currently supplied by the sensing member. In thepresent example the comparison is executed every 100 μsec. Thecomparison itself is for example realised based on a greyvalue;

3. If the current intensity value is higher than the stored one, thecurrent intensity value is stored into the memory element overwritingthereby the stored value. Each time a storage operation is performedsubsequent to a comparison, then the current scanning signal is alsostored in order to timestamp the stored intensity value. If on the otherhand the current intensity value is lower, or even equal, to the storedone, then the stored intensity value remains unchanged;

4. At the end of the scan, the stored scanning signals are readout fromeach of the processing elements. Since those scanning signals correspondto the position where a maximum intensity value was obtained, theposition of the different points on the object is easily determined;

5. Optionally the stored maximum intensity values could also be readout.This would provide a standard video image of the object surface 6, takenunder telecentric light condition.

There are several possibilities to physically implement the processingelements and the sensor elements. However at least 1000 processingelements would be required. With a data rate of one pixel per 100 nseci.e. 10 MHz, the greyvalues of the pixels of one column could beprocessed. A digital processor with a clockrate of a few hundred MHzoperating with a data memory of 1000 running maximum values and 1000timestamp registers would perform the algorithm described here before.By using a higher pixel clock rate than 10 MHz, it would be possible tohave several columns of the sensing elements share a same processingelement.

Other implementations could also be used. For example instead of storingthe maximum intensity value as a digital value, an analogous storagecould be considered such as for example a voltage stored in a capacitor.The incoming pixel greyvalue corresponding to the light intensity wouldthen also be supplied as a voltage value. The comparison operation suchas presented under step 2 would then be executed as an analogue voltagecomparison. The storage of the scanning signal providing the timestampcould be dynamic, i.e. one capacitor per bit. The decision step (3)would then be implemented as a set of switches controlled by thecomparator. If the running maximum is to be updated, the switches leadthe current of the running greyvalue to the capacitor and the currentscanning signal to the local timestamp registers.

An alternative approach to the method described here before and using anintelligent CMOS sensor, is to use a parallel-readout very fast standardCMOS sensor without on-chip processing. This alternative is illustratedin FIG. 7. Assuming for example 64 parallel readout channels with a rateof 50 MHz each, the cumulated data rate is 3.2 billion pixels persecond. In the present context this is to be regarded as rather slow.Using this sensing principle, one would weaken the confocal principle byreducing the numerical aperture of objective 5. This would result in abroader maximum of light intensity per pixel as a function of z positionof pinhole array 4, for example like the dotted curve in FIG. 2. Due tothe broader maximum, less dense sampling of z-positions would befeasible, with computational interpolation in between sampled z-positionin order to achieve the desired few micron accuracy. Here, with about100 samples at 30 μ z-spacing due to numerical interpolation one couldalso achieve few micron accuracy. Taking 100 samples at 3.2 billionpixels per second takes about 32 msec. This is compatible with therequirements of in-line measurement. The drawback of this method is theenormous volume of electronic equipment required to deal with the 64parallel readout channels: 64 digitizers, 64 memory banks, and 64processors are needed to do the required maximum calculation andinterpolation. That is, one essentially needs 64 complete imageprocessing systems; otherwise, the processing of 100 million pixelsrequired in this approach would take too much time violating therequirements for in-line measurement. This complexity could be tradedagainst acquisition time. With 16 channel parallel readout in aboveexample 128 msec can still be achieved which is quite acceptable. Thememory banks have to contain the data which are required for thenumerical interpolation for each separate pixel. The size of the memorybanks can be limited by storing only the intensity values in theneighbourhood of the maximum intensity, the maximum intensity itself andthe timestamp (only 1 byte in this case because the number of z-slicesis 100) corresponding to the maximum value for each pixel.

Suppose that five sample points (pixel greyvalues for different zpositions) are used as data enabling an interpolation in order to obtaina maximum intensity value corresponding to the surface z-value. Becausethe scanning member operates mechanically, the pixel values arrive as atime sequence. Assume a z-scan of 100 images, the five sample pointsneed to be found around the global maximum intensity value within theseries of hundred values, independently for each x-y pixel location.

FIG. 5 illustrates an example of a confocal z-response curve for asingle xy-pixel. As can be seen in this FIG. 5, there is a local maximumat z1 preceding the global maximum at z2 within the scan sequence.Because of side-lobes in the confocal response curve such local maximumare not exceptions and should be dealt with.

Since the time sequence corresponds to the z sequence or scanningdirection, it may happen that five greyvalues (a-e) and theircorresponding timestamp (z1) are stored in the memory whereas theycorrespond to the local and not to the global maximum. The subsequenttwo greyvalues (f and g) cause then a problem as it is not clear whetherthey belong to the global maximum or not. If they would not belong tothe global maximum they could simply be ignored because they have alower value than the greyvalues b, c or d. However if the greyvalue of fand g is required because they are part of the global maximum (f-k, z2),then they can't be ignored. Thus even if only five sample points areused, it is nevertheless necessary to foresee at least nine memoryaddresses (A, B, C, D, E, F, G, Z, J) five (A, B, C, D, E) for thecurrent running local maximum, two (F, G) for the last two in the timesequence and one for a z-bit (Z) and another one (J) for a history flagas will be described hereinafter. The image processing member furtherneeds an image sequence counter i which is common for all xy pixelswithin the field-of-view.

The processing applied by the image processor uses an algorithm which isillustrated in FIG. 8 and will be described by using table I shown inFIG. 10.

As illustrated in FIG. 8, the algorithm comprises the following steps:

-   50.NSS: each time a new series of scanning signals is started, the    algorithm is initialised by each of the image processing elements.-   51.A= . . . J=0; i=0: each of the memory storage locations, A, B, C,    D, E, F, G, Z and J, as well as the image sequence counter i are set    to an initial value for example 0. This is illustrated in the second    row of table I.-   52 RDGV=p: the actually supplied greyvalue p, such as supplied by    the sensing element is read by the image processing element for    further processing.-   53 i=i+1: the counter i is incremented by one unit as illustrated in    column 1 of table I.-   54 RDC: the value stored at memory location c is read.-   55 p>cont c: the actual greyvalue p is compared with the greyvalue    stored at memory location c. If p has a higher value (y) than the    one stored at memory location c, then there is stepped to step 56.    If on the other hand p has a lower or an equal value (N) than the    one stored at memory location c, then step 57 is executed.-   56 ST NM: because the actual greyvalue p was higher than the one    stored at memory location c, this signifies that probably a new    maximum value has been recognized, which implies a write operation    into the memory location c. The following write operation is then    performed A=F, B=G, C=p, Z=i, J=2, F=G and G=p. More details about    this particular operation as well as of other write operations    described in the subsequent steps will be given in the example    described hereunder and referring to FIG. 5 and table I.-   57 STNNM: because the actual greyvalue is lower than the one stored    in memory location c, this actual greyvalue can not be considered as    a maximum. The following write operation is then performed F=G, G=p.-   58 J=2?: there is verified if J=2, indicating that during the    preceding storage step a presumed maximum was found.-   59 ST1: p is stored in memory location D (D=p) and J is decremented    (J=J−1) if during step 58 J=2.-   60 ST2: p is stored in memory location E (E=p) and J is decremented    J=J−1 if during step 58 J≠2.-   61 LST SC ?: there is verified if the actual scanning signal was the    last one of a series of scanning signals.-   62 STP: the routine executing the algorithm is finished if the    actual scanning signal was the last one of the series.

Referring back to FIG. 5 and row three of table I, one can observe thatif the actual received value p=a, i.e. the first greyvalue shown in FIG.5, the counter i=1 since it is the first pixel value received. As atstorage location C a greyvalue 0 is stored (C=0) and as a>0 (step 55),step 56 is executed. This means that at storage location C, the value ais stored (C=a). The values stored at F and G are shifted to A and Brespectively, i.e. A=0, B=0. As a maximum is presumed J=2 and the valuestored at G is shifted at F, i.e. F=0 whereas the actual greyvalue a isalso stored at G (G=a). The fact that G is shifted at F and G=p is toenable to make a distinction between a local and a global maximum.Therefor, besides the five memory locations A to E, also F and G arereserved. Finally Z=1 indicating that at the first one of the scanningsignals a maximum is presumed.

The next greyvalue i.e. greyvalue b is now considered and counter i isincremented to i=2 as is shown in the fourth row of table I. Since C=aand since b>a (step 55) the greyvalue is stored at C respectively G isoverwritten by b (C=b). Greyvalue a stored at G is stored at B (B=a) andF=a. As greyvalue b is again a presumed maximum Z=i and thus Z=2 andJ=2.

At the fifth row of table I the subsequent greyvalue c is considered. AsC=b and c>b an analogous operation as described here before is executedleading to: i=3, A=a, B=b, C=c, F=b, G=c, Z=3, J=2.

The subsequent greyvalue d (see the sixth row of table I) will now beconsidered. Counter i is set to i=4 and since C=c and d<c (see FIG. 5),d is not considered as a maximum and therefor there is stepped to step57 where F=G and G=p, i.e. F=c and G=d. Subsequently at step 58 there isestablished that the actually stored value at J=2 so that there isswitched to step 59 where D=p as subsequent greyvalue and J=J−1 leadingto D=d and J=1. As no new maximum greyvalue is found, Z remainsunchanged.

Greyvalue e following greyvalue d is also smaller than greyvalue cstored at memory location C. Therefor step 57 is again executed leadingto F=d and G=e. However at step 58, there is now established that J≠2(J=1) so that there is continued with step 60. At this step J isdecremented to J=0 and E=e as subsequent greyvalue. Again Z remainsunchanged as no new maximum is recognised.

Reaching the next greyvalue f (i=6) there is established that f<c sothat F=e and G=f. As J≠2 and J=0, J is not further decremented sinceonly two greyvalues beyond the actual maximal value (c in the presentexample) are considered. J has a flag function enabling to identify theposition of the considered greyvalue with respect to the actual maximum.Byte Z remains further unchanged as no new maximum is recognised.

The subsequent greyvalue to be considered is g (i=7). Although g>f, g isstill smaller than the greyvalue c stored at C. Therefor F=f, G=g, J=0and z remains Z=3 (steps 57, 58 and 60). Reaching however greyvalue h(i=8) this greyvalue is larger than greyvalue c stored at memorylocation C, so that at step 55 there is switched to step 56 leading to:A=f, B=g, C=h, J=2, F=g, and G=h. Since a new maximum is found, Z has tobe adapted to Z=8.

The subsequent greyvalue J (i=9) is smaller than h, the actual maximumstored at memory location C. At step 57 F=h and G=j and at step 58 thereis established that J=2. Therefor there is switched to step 59 whereJ=2-1 or J=1 and D=j. Z remains Z=8 is no new maximum recognised.Finally with greyvalue k (i=10) there is established that k<h and thatJ≠2. Therefor J is decreased to J=0 and F=j, G=k and E=k.

This signifies that at locations A to E the greyvalues f to krepresenting the global maximum are now stored and not a to erepresenting a local maximum. The present algorithm thus enables torecognise a local and a global maximum from one another and retain onlythe global maximum as confocal response signal. Moreover, by the storageof the Z value it is possible to retain also that scanning signal amongthe considered series that has led to the considered maximum and thus todetermine the position of the mask and derive therefrom the required Zposition of the object 6.

Once the scanning operation is determined and the necessary greyvaluesare stored in the memory locations A to E and the maximum at 2, aninterpolation operation is required to obtain the correct maximum value.Different algorithms can be used for this purpose. One possiblealgorithm is power-weighted-center-of-gravity

${Z\mspace{14mu}\max} = \frac{\sum\limits_{u = 1}^{5}{\left( {{gv}(u)} \right)^{2} \cdot u}}{\sum\limits_{u = 1}^{5}\left( {{gv}(u)} \right)^{2}}$

where u represents the grevalues gv stored at the five memory locationsA to E (A=1 , . . . , E=5).

This method provides similar output as the one using a processingelement per sensing element. For each pixel one byte for the maximumintensity value and two bytes for the corresponding interpolatedz-position (2 bytes are required because of the interpolation). Similarto the previous method, two maps can therefor be transferred to an imageprocessing board, that can be situated externally or also integratedwithin the camera itself. One map contains the maximum intensity valuesfor each pixel and another map contains the surface topography.

Another possible variation is the additional use of a 3D camera as astandard 2D camera. It might be considered useful that one coulddisregard the maximum formation and simply integrate pixel intensitiesover some time period corresponding to a standard integrating CMOScamera. Within one and the same mounting position within somemanufacturing equipment one could thus acquire 2D or 3D images asdesired.

The opposite choice also is possible. If readout of maximum greyvalue asdiscussed above is undesirable (for silicon layout or other specificimplementation-related reasons for example) and the intelligent CMOSsensor would produce only the position of maximum, one could add asecond beamsplitter into the arrangement of FIG. 1 and add a standardCMOS or CCD camera after this second beamsplitter. This camera wouldintegrate light over the time of the entire z-scan. In this way thestandard 2D image is obtained by this extra standard camera.

A further variant of the measurement device is obtained if light source1 is replaced by a light source close to the object 6. In this case theset-up loses its confocal property and becomes a “depth through focus”sensor. Also, the image comprised of the maximum greyvalues during thez-scan corresponds to the normal 2D image for this second form ofillumination. This might be useful since in this way images of object 6may be acquired with differing types of illumination.

1. A device for measuring in three dimensions a topographical shape ofan object, said device comprising an arrayed confocal imaging systemhaving a light source provided for illuminating said object and a lightpath diverging optical element, provided for diverging a path ofilluminating light output by said light source and a path of reflectedlight reflected by said object, said confocal imaging system furthercomprises a confocal topographical mask provided for convertingilluminating light coming out from said light path diverging opticalelement into an array of small spots, said confocal imaging systemfurther comprises a confocal objective provided for orienting saidilluminating light towards said object and said reflected light towardssaid confocal topographical mask in order to form a confocal image, saiddevice further comprises a photoelectric sensor member provided toreceive said reflected light having passed said confocal topographicalmask and deflected by said light path diverging optical element and toconvert the latter light into an intensity value, said device comprisingalso a scanning member on which said confocal topographical mask ismounted, said scanning member being provided for moving said confocaltopographical mask over successive positions over a predetermineddistance in order to modify a relative distance in a predetermineddirection between said object and the object-position-in-focus, saiddevice further comprises an image processor connected to saidphotoelectric sensor member and provided for forming a confocal responsesignal and calculating said object shape from said confocal imagesacquired at different relative distances in said predetermined directionbetween said object and the object-position-in-focus by saidphotoelectric sensor member, said confocal objective being provided formapping at successive object-position-in-focus said array of small spotsoutput at said successive positions, characterised in that said confocalobjective is a 3D confocal objective mounted at a fixed position withinsaid device.
 2. A device as claimed in claim 1, characterised in thatsaid 3D confocal objective is being designed in such a manner as tolimit spherical aberration and coma in order to keep imaging errors insaid confocal images at a maximum of two pixels.
 3. A device as claimedin claim 1, characterised in that said confocal topographical mask isformed by a microlens array mounted on said scanning member and saiddevice further comprises a single pinhole located at a focal point ofsaid photoelectric sensor member.
 4. A device as claimed in claim 3,characterised in that said single pinhole is located at a focal point ofa two-stage camera objective placed in front of said photoelectricsensor, said single pinhole being mounted between said two-stage cameraobjective.
 5. A device as claimed in claim 3, characterised in that saidsingle pinhole is located in an optical path of said reflected lightbetween a two-stage objective forming a sensor objective.
 6. A device asclaimed in claim 1, characterised in that said confocal topographicalmask is formed by a pinhole array, each pinhole being provided with amicrolens.
 7. A device as claimed in claim 1, characterised in that saidillumination source comprises a high-density LED array with a furthermicrolens bonded on top of it.
 8. A device as claimed in claim 1,characterised in that said illumination source comprises an array oflaser diodes having an output coupled into a fibre-optic bundle.
 9. Adevice as claimed in claim 1, characterised in that a linear polarizeris applied into said light path at an illumination side of saiddiverging optical element on which other side a quarter-wave plate andan analyser are applied.
 10. A device as claimed in claim 1,characterised in that said scanning member comprises a scanning signalgenerator provided for generating a series of scanning signalsindicative of said successive positions, said scanning signal generatorbeing connected to said image processor for supplying said scanningsignals thereto, said photoelectric sensor member comprising a firstarray of sensing elements, said image processor comprises a second arrayof processing elements, each processing element of said second arraybeing each time connected to a number of sensing elements of said firstarray, said image processor being provided for receiving each timewithin a same series of scanning signals successive ones of saidintensity values, each of said processing elements being provided fordifferentiating said successive intensity values among each other andretaining those intensity values forming said confocal response signal,said image processor being provided for associating to those intensityvalues, forming said confocal response signal, those scanning signalsrepresenting those positions having led to said confocal responsesignal.
 11. A device as claimed in claim 10, characterised in that asampling member is mounted between said first array of sensing elementsand said second array of processing elements, said sampling member beingprovided for sampling at a predetermined sampling rate said intensityvalues and output at parallel read-out gates of said sensing elements,each of said processing elements having a memory element provided forstoring intensity values sampled within a same series of scanningsignals, each of said processing elements being provided for determininga maximum intensity value by interpolating the intensity values storedeach time in a same one of said memory elements.
 12. A device as claimedin claim 10, characterised in that said second array has at least a samenumber of elements as said first array, each processing element beingprovided for storing as a stored intensity value in an associated memoryelement an initial intensity value, said processing elements beingprovided for comparing, under control of each of the subsequent scanningsignals, if the current intensity value is higher than the storedintensity value and for overwriting the stored intensity value if thelatter is lower than the current intensity value, said processingelements being also provided for storing upon each storage operation thecurrent one of the scanning signals.
 13. A device as claimed in claim 1,characterised in that said scanning member comprises a voice-coilactuator connected to said confocal topographical mask and provided forimposing a linear movement to said mask.
 14. A device as claimed inclaim 13, characterised in that said voice-coil actuator is connected toan optical encoder provided to monitor said movement and for generatinga displacement signal thereof, said optical encoder being connected tosaid scanning signal generator which is provided to generate saidscanning signals from said displacement signal.
 15. A device formeasuring in three dimensions a topographical shape of an object, saiddevice comprising an arrayed confocal imaging system having a lightsource provided for illuminating said object and a light path divergingoptical element, provided for diverging a path of illuminating lightoutput by said light source and a path of reflected light reflected bysaid object, said confocal imaging system further comprises a confocaltopographical mask provided for converting illuminating light coming outfrom said light path diverging optical element into an array of smallspots, said confocal imaging system further comprises a confocalobjective, provided for orienting said illuminating light towards saidobject and said reflected light towards said confocal topographical maskin order to form a confocal image, said device further comprises aphotoelectric sensor member, provided to receive said reflected lighthaving passed said confocal topographical mask and deflected by saidlight path diverging optical element and to convert the latter lightinto an intensity value, said device comprising also a scanning memberprovided for modifying a relative distance in a predetermined directionbetween said object and the object-position-in-focus, said devicefurther comprises an image processor connected to said photoelectricsensor member and provided for forming a confocal response signal andcalculating said object shape from said confocal images acquired atdifferent relative distances in said predetermined direction betweensaid object and the object-position-in-focus by said photoelectricsensor member, said confocal objective comprises a first and a secondpart, said second part being fixed whereas said first part is mounted onsaid scanning member which is provided for moving said first part oversuccessive positions in said predetermined direction over apredetermined distance, said first part being mounted near to saidobject, said confocal objective being provided for mapping at successiveobject-position-in-focus said array of small spots output at saidsuccessive positions characterised in that said confocal topographicalmask is formed by a microlens array and said device further comprises asingle pinhole located at a focal point of said photoelectric sensormember.
 16. A device as claimed in claim 15, characterised in that atelecentric aperture is mounted between said first and second part. 17.A method for measuring in three dimensions a topographical shape of anobject by means of an arrayed confocal imaging system, said methodcomprising an illumination of said object through a light path divergingoptical element provided for diverging a path of illuminating light anda path of reflected light reflected by said object, said path ofilluminating light crossing a confocal imaging system comprising aconfocal topographical mask converting illuminating light coming outfrom said light path diverging optical element into an array of smallspots, said small spots being oriented towards said object by a confocalobjective which further orients said reflected light towards saidconfocal topographical mask in order to form a confocal image, saidconfocal image being supplied to a photoelectric sensor member afterhaving passed said confocal topographical mask and being deflected bysaid light path diverging optical element, said photoelectric sensorconverting the latter light into an intensity value, said method alsocomprises a scanning operation comprising a movement of said confocaltopographical mask over successive positions in a predetermineddirection over a predetermined distance in order to modify a relativedistance in said predetermined direction between said object and theobject-position-in-focus, said method further comprises an imageprocessing wherein a confocal response signal is formed from datasupplied by said photoelectric sensor member and wherein said objectshape is calculated from said confocal images acquired at differentrelative distances in said predetermined direction between said objectand the object-position-in-focus by said photoelectric sensor member,said confocal objective maps at successive object-position-in-focus saidarray of small spots output at said successive positions, characterisedin that said confocal objective remains fixed during said scanningoperation.
 18. A method as claimed in claim 17, characterised in thatsaid scanning operation comprises a generation of a series of scanningsignals indicative of said successive positions and a supply thereof tosaid image processor, said image processing comprises a receipt, eachtime within a same series of scanning signals, of successive ones ofsaid intensity values, said processing comprises a differentiating ofsaid successive intensity values among each other and a retaining ofthose intensity values forming said confocal response signal, said imageprocessor also comprises an association to those intensity values,forming said confocal response signal of those scanning signalsrepresenting those positions having led to said confocal responsesignal.